Activity of Benomyl for Control of Postbloom Fruit Drop of Citrus Caused by Colletotrichum acutatum.

Postbloom fruit drop (PFD) of citrus caused by Colletotrichum acutatum produces orange-brown lesions on petals and induces the abscission of young fruitlets and the retention of the calyces. Despite the fact that C. acutatum is not highly sensitive to benomyl in culture, this fungicide provides good control of the disease under field conditions. This study was undertaken to determine the effect of benomyl on various stages of disease development to understand the basis for its effectiveness in the field. We found that benomyl at 1.0 µg/ml reduced colony area of C. acutatum by about 75% and completely inhibited growth of C. gloeosporioides. Benomyl did not prevent conidial germination even at 100 µg/ml, but reduced germ tube elongation at 10 and 100 µg/ml. When benomyl was applied to flower clusters on screenhouse-grown plants before inoculation, disease severity was greatly reduced. Applications at 24 and 48 h, but not at 72 h, after inoculation reduced PFD severity. Application of benomyl to symptomatic petals not bearing conidia did not prevent or reduce production of inoculum. Application to petals bearing conidia reduced viability of these fungal propagules by only about 50%. The viability of appressoria on mature leaves was not affected by benomyl application. Even when appressoria on mature leaves were stimulated to germinate by treatment with flower extracts, subsequent application of benomyl did not reduce propagule numbers below original levels. Benomyl appears to act by preventing infection and early development of the fungus in petals. However, once symptoms have developed, this fungicide has only minimal effects on further disease development and spread.

Environmental Factors Affecting the Severity of Alternaria Brown Spot of Citrus and Their Potential Use in Timing Fungicide Applications.

Alternaria brown spot affects many tangerines and their hybrids, causing lesions on leaves, twigs, and fruit resulting in reduced yield and fruit quality. Field studies were conducted in a severely affected Minneola tangelo grove in central Florida from 1996 to 1998 to determine the environmental factors associated with infection of field trees and potted trap plants. Conidial production peaked following large flushes of new leaves, which were heavily infected. Most infections occurred during the summer rainy season, but trap plants became infected nearly every week of the year. When plants were exposed for 1-week periods, linear regression analysis indicated that disease severity on trap plants was positively related to the amount of rainfall, duration of leaf wetness, and average daily temperatures, and negatively related to the number of conidia trapped. Similar relationships occurred with trap plants exposed for 24-h periods on 141 different dates, except that temperature was not a significant factor. Nevertheless, these factors individually or combined in stepwise multiple regressions explained only a low percentage of the variability in disease severity with both weekly and daily trap plant sampling. When daily environmental data were categorized as: (i) rain versus no rain, (ii) <10 h or >10 h leaf wetness duration, and (iii) average temperature <20°C, 20 to 28°C, and >28°C, relationships to disease severity on trap plants were clearer. Disease severity on days with rain was nearly double that of days without rain, but considerable infection occurred on days with >10 h leaf wetness duration and no rain. Infection was greatest on days with temperatures of 20 to 28°C and slightly less at lower or higher temperatures. A point system, called the ALTER-RATER, was designed whereby each day would be assigned a severity value according to the prevailing environmental conditions. A fungicide application would be made after a predetermined number of points had accumulated. Simulated spray programs based on accumulation of 50, 75, 100, and 150 points from historical weather data at several locations indicated that from 8 to 15, 6 to 8, 5 to 6, or 3 to 4 sprays, respectively, would be needed depending on year and location in Florida. Such a weather-based control system could reduce the number of fungicide applications and improve control of Alternaria brown spot of tangerine.

Phytophthora Brown Rot of Citrus: Temperature and Moisture Effects on Infection, Sporangium Production, and Dispersal.

Sporadic epidemics of Phytophthora brown rot occur in citrus orchards in Florida and are caused primarily by Phytophthora palmivora and secondarily by P. nicotianae. With P. palmivora, fruit wetness durations of 3 h or more resulted in maximum infection at favorable temperatures. The optimum temperature range for fruit infection and brown rot development was 27 to 30°C. No brown rot developed at 22°C or less. The optimum temperature for sporulation in culture and on the fruit surface was 24°C, with sporangium production decreasing rapidly at higher or lower temperatures. A few sporangia were produced with 18 h of fruit wetness, and numbers increased as duration of wetness increased up to 72 h. Propagules were splash-dispersed by single droplets of water from infested peel disks horizontally about 350 to 450 mm and vertically about 30 to 45 mm with most isolates. Sporangia were not dispersed by air currents alone. Few sporangia of P. nicotianae were produced on fruit surfaces, and they were not readily dispersed by falling water droplets. Brown rot caused by P. palmivora can become epidemic in Florida from July to October during the rainy season because of its high temperature optimum and prolific sporulation on the fruit surface.

Metalaxyl-Resistant Isolates of Phytophthora nicotianae: Occurrence, Sensitivity, and Competitive Parasitic Ability on Citrus.

Phytophthora nicotianae was recovered from 8 of 11 nursery sites, and metalaxyl-resistant isolates were detected at 6 of the 8 positive sites. Resistant isolates comprised 31 to 59% of the population at those sites. Most resistant isolates grew well in the presence of 100 µg/ml of metalaxyl, whereas sensitive isolates did not grow at 1.0 µg/ml. Some isolates from mature citrus groves were intermediate in their sensitivity. Metalaxyl-drench treatments failed to control resistant isolates in greenhouse tests, but fosetyl-Al was usually effective. The replacement series technique was used to compare the relative competitive ability of resistant and sensitive isolates in greenhouse tests. Results varied according to the isolate used, but at least some metalaxyl-resistant isolates were able to compete favorably with sensitive isolates in the absence of metalaxyl. In a field test, a high percentage of the population remained resistant to metalaxyl even after 2.5 years without treatment with this fungicide. Excessive use of metalaxyl in nursery situations has reduced the future utility of the product in Florida citrus groves.

Split Applications of Copper Fungicides Improve Control of Melanose on Grapefruit in Florida.

Citrus melanose, caused by Diaporthe citri, is a major disease of fresh market grapefruit which is controlled primarily by preventive applications of copper fungicides. The goal of this research was to improve melanose control without increasing the total amount of metallic copper utilized. Disease control was assessed in field experiments for 3 years using one, two, three, or four applications of copper fungicides in which the same total amount of metallic copper was applied per season in each program. Melanose was severe in 1995 and 1997, and the four-application schedule provided better control than the one-, two-, or three-application programs. In 1996, disease pressure was low and all programs and fungicides provided equally effective control. Analysis of copper residues on the fruit indicated that loss of protection was attributable more to increase in fruit surface area when fruit were growing rapidly than to removal of copper by weathering. On larger, slower-growing fruit, loss of copper residues was apparently due to both factors. Economic evaluation of improved packouts of fresh market fruit and additional application costs indicated that net returns would be increased by $650 to $1,250 per hectare during years in which melanose was severe when the four-spray program was compared to a single spray.

The Role of Colletotrichum spp. in Postharvest Anthracnose of Citrus and Survival of C. acutatum on Fruit.

Colletotrichum gloeosporioides causes postharvest anthracnose of citrus fruit and is a common saprobe in citrus groves, whereas C. acutatum infects flower petals and causes postbloom fruit drop (PFD). The purpose of this study was to determine whether C. acutatum caused anthracnose, survived as quiescent infections on fruit, and represented a threat for introduction of the PFD pathogen to disease-free areas. When Robinson tangerine fruit were artificially inoculated with both species and exposed to high levels of ethylene, only fruit inoculated with C. gloeosporioides developed postharvest anthracnose. However, C. acutatum was readily reisolated from inoculation sites on asymptomatic fruit. Navel oranges harvested from groves with severe PFD the previous spring developed low levels of anthracnose, but only C. gloeosporioi-des was isolated from lesions. Valencia orange fruit harvested at bloom during a PFD outbreak had high surface populations of C. acutatum which declined rapidly during 2 to 3 weeks of storage. Quiescent infections formed on artificially inoculated and naturally exposed fruit were reduced but not eliminated by washing, waxing, and fungicide treatment in the packinghouse. Since C. acutatum does not produce postharvest anthracnose, the risk of introduction of this pathogen to disease-free areas is minimal. However, if fruit were harvested during a PFD outbreak, C. acutatum might survive standard packinghouse treatment and shipment as quiescent infections.

Environmental Factors Affecting Production, Release, and Field Populations of Conidia of Alternaria alternata, the Cause of Brown Spot of Citrus.

ABSTRACT Alternaria brown spot, caused by Alternaria alternata pv. citri, affects many tangerines and their hybrids, causing loss of immature leaves and fruit and reducing the marketability of the remaining fruit. Conidial production of A. alternata was greatest on mature leaves moistened and maintained at near 100% relative humidity (RH) for 24 h, whereas leaves that had been soaked or maintained at moderate RH produced few conidia. Conidial release from filter paper cultures and infected leaves was studied in a computer-controlled environmental chamber. Release of large numbers of conidia was triggered from both substrates by sudden drops in RH or by simulated rainfall events. Vibration induced release of low numbers of conidia, but red/infrared irradiation had no effect. In field studies from 1994 to 1996, air sampling with a 7-day recording volumetric spore trap indicated that conidia were present throughout the year with periodic large peaks. The number of conidia captured was not closely related to rainfall amounts or average wind speed, but was weakly related to the duration of leaf wetness. Likewise, disease severity on trap plants placed in the field weekly during 1995 to 1996 was not closely related to conidial numbers or rainfall amounts, but was weakly related to leaf wetness duration. Sufficient inoculum appears to be available to allow infection to occur throughout the year whenever susceptible host tissue and moisture are available.